The invention relates generally to the storing of digital video streams and more particularly to the storing of uninterrupted digital video streams using a motion-sensitive memory system, such as an optical memory system.
Present electronic devices allow for the recording of video data in a digital format. A common format for recording digital video data is defined by the Motion Picture Expert Group (MPEG) and is known as the MPEG format. Because digital video data is extremely voluminous, the MPEG format allows captured digital video data to be compressed before it is stored. Digital video data is stored on a variety of media, including magnetic tapes, magnetic disks, and optical disks. In the optical storage field, digital video data is commonly stored on read only memory (ROM) disks, such as digital video disks (DVDs).
Current advances in the optical storage field include optical memory systems that allow writing and/or rewriting of data onto optical disks. In writing data to an optical disk, a write head directs a beam of laser light into narrow recording tracks on the optical disk. Unwanted motion of the write head during writing can cause the laser beam to improperly contact the desired recording track, thereby degrading the quality of the recorded data. Because of the motion sensitivity of optical writing systems, optical writing systems are typically employed only in stable devices, such as desktop computers.
As previously noted, video images captured in digital video format require a large volume of storage memory. Because of the large storage capacity of optical storage systems, it is desirable to optically record or write digital video data onto an optical disk. For example, a handheld video camera equipped with an optical memory system that writes data onto an optical disk provides the ability to capture a large volume of video data without having to repeatedly exchange the storage media.
One problem with writing digital video data to an optical disk incorporated into a handheld video camera is that optical memory systems are motion-sensitive and a handheld camera is susceptible to motion caused by the user or the environment of the user (i.e., recording digital video data while traveling in a moving car). FIG. 1 is a depiction of the components of a handheld digital video camera system 10 that records digital video data onto an optical memory disk. The digital video capture device 12 is a conventional video camera that combines optical lenses and a charge coupled device (CCD) array to electronically capture image data in a digital format. Digital video data that is captured in the digital video capture device is forwarded to a compression unit 14 that compresses the digital video data using conventional compression techniques. The compressed digital video data is forwarded to a write buffer 16 that buffers the compressed digital video data in order to smooth out the delivery of the compressed video data to an optical memory system 18.
The write buffer 16 is typically made up of a fixed amount of RAM memory that is incorporated into the video camera circuitry. The conventional optical memory system includes a write head and an optical disk spindle. The write head includes a laser source that is used to optically write digital video data to the narrow tracks of an optical disk that is located on the optical disk spindle. As stated above, the writing process is a very motion sensitive and precise operation. If mechanical disturbances beyond a certain threshold are encountered by an optical memory system during data writing, it is likely that the data writing will be adversely affected. For example, when data writing is continued during a mechanical disturbance, video data recorded on adjacent tracks of the disk may be destroyed or rendered unreadable. In order to minimize the problems involved with writing to an optical memory system, it is desirable to momentarily stop recording when unacceptable mechanical disturbances, measured as motion or acceleration, are detected by an acceleration detection device, such as an accelerometer 20.
While momentarily stopping writing to an optical disk during periods of mechanical disturbances avoids recording inaccurate digital video data and/or destroying data on adjacent tracks, possibly rendering the disk unreadable, a different problem may be created if data writing is suspended for too long a period of time. The problem involves overflowing the write buffer 16 that is located between the compression unit 14 and the optical memory system 18. Assuming a continuous stream of digital video data is being generated by the video capture device 12 during the time that writing to the optical memory system is suspended, the write buffer is accumulating the digital video data that cannot be written to the optical memory system. If the write buffer reaches its storage capacity, instead of having unreliable data recorded because of mechanical disturbances, complete portions of digital video data will be lost as the write buffer begins to drop data for which it has no storage capacity.
Prior art solutions to the problem of writing data in spite of mechanical disturbances include increasing the size of the write buffer 16 and/or increasing the write head tracking bandwidth of the optical memory system 18. For example, the write buffer can be made sufficiently large such that digital video data captured during a mechanical disturbance is absorbed into the large write buffer until the optical memory system can resume recording. While increasing the capacity of the write buffer may work well, it may not be cost-effective to simply add RAM to the write buffer. Regarding increasing the write head tracking bandwidth of the optical memory system for the purpose of solving the writing problem, additional mechanical stabilization would likely have to be added to the memory system along with feed forward control of the write head actuator based on input from the accelerometer to stabilize the write head and the optical disk. Again, this adds to the expense of the video system and may not be cost-effective.
In view of the stated shortcomings of the techniques used to overcome the problems involved with storing digital video data with motion-sensitive memory systems, what is needed is a method and apparatus that can store uninterrupted streams of compressed digital video data even when mechanical disturbances cause data writing to be temporarily interrupted.
It is important to note that there is a large volume of prior art that deals with increasing the capture rate for recording video data that includes images in a first pace motion, such as a horse running in front of a fixed landscape, and that deals with minimizing the shake that is perceived in a video stream that is recorded with an unsteady camera. Although prior art concerning these two concerns may seem related to the present disclosure, this prior art is only concerned with how video data is captured and compressed, rather than how the video data is transferred to a long-term storage medium, such as an optical disk.
A method and apparatus for storing an uninterrupted digital video stream with a motion-sensitive memory system involve monitoring the acceleration experienced by the memory system and adjusting the compression of the digital video stream in response to the acceleration. In the preferred embodiment, a threshold-related condition is established such that detection of the condition simultaneously triggers a suspension of data writing and an increase in a data compression ratio. As long as an acceleration threshold is not exceeded, the compressed digital video data is written to an optical disk by an optical memory system. However, when an occurrence, such as a mechanical disturbance, causes the acceleration threshold to be exceeded, data writing is suspended, because the acceleration jeopardizes the reliability of the data writing. When acceleration causes data writing to be suspended, the compression of the digital video data is increased so that the digital video is not lost while it continues to be captured by a video capture device.
In the preferred embodiment of the invention, the uninterrupted digital video streams are captured and stored with a handheld digital video camera that records compressed digital video data with an on-camera optical memory system that utilizes an optical disk, such as an 8 cm digital video disk (DVD). The components of the digital video system include a video capture device, a compression unit, a write buffer, an optical memory system, an acceleration detection unit, and a compression controller. The video capture device utilizes optical lenses to focus light from objects of interest onto an array of charged coupled device (CCD) sensors. The CCD array outputs digital data that is formatted into digital video data according to the MPEG II standard and the quality of the digital video data corresponds to the resolution of the CCD array and the speed with which video data is extracted from the CCD array.
The compression unit receives digital video data from the video capture device and compresses the digital video data to maximize the amount of data that can be stored by the associated optical memory system. The compression unit compresses digital video data at a particular compression ratio, where the compression ratio is defined as the volume of compressed data that is generated from a given volume of uncompressed data.
The write buffer is memory that temporarily stores compressed digital video data in order to average the transfer rate of the data that is forwarded to the optical memory system. The write buffer includes RAM memory that is sufficiently large to store digital video data that accumulates during temporary interruptions of data writing to the optical memory system.
The optical memory system includes a write head and an optical disk, where the write head utilizes a laser source that generates a narrow beam of laser light to write digital data into tracks of the optical disk. In order to accurately generate the physical changes in an optical disk at the required data density, the laser beam must be directed onto the optical disk with extreme accuracy. If mechanical disturbances create acceleration of the optical memory system beyond the established acceleration threshold, data writing must be temporarily suspended until acceleration is within the acceptable parameters. Data writing must be temporarily suspended because the writing of the data to the optical disk becomes unreliable as a result of unpredictable motion between the write head and the optical disk.
The acceleration detection unit is used to detect acceleration of the digital video system, and specifically of the optical memory system. The acceleration detection unit may include an accelerometer that directly senses acceleration of the optical memory system. Alternatively, the acceleration detection unit may include specialized circuitry that analyzes captured video frame data to calculate acceleration of the optical memory system.
The interaction of the compression controller, the acceleration detection unit, and the compression unit is the focus of the invention. The compression controller includes logic that is operatively connected between the acceleration detection unit and the compression unit. The function of the compression controller is to indicate to the compression unit that the digital video compression ratio should be adjusted to account for time periods when writing to the optical memory system has been suspended because of occurrences, such as mechanical disturbances, that impose conditions in which the acceleration threshold is exceeded. In order to influence the compression unit, the compression controller receives input data from the acceleration detection unit concerning any accelerations that the optical memory system is being subjected to. If the optical memory system is being subjected to accelerations greater than the pre-established acceleration threshold, the compression controller calculates the adjustments in the compression ratio that are necessary to compensate for the time that writing by the optical memory system is suspended. The goal of the adjusted compression ratio is to ensure that the write buffer does not overflow with digital video data that is streaming from the video capture device while writing to the optical memory system is suspended. If the write buffer were allowed to overflow with streaming digital video data, video data would be permanently lost and viewed video images may look choppy and/or incomplete.
The compression controller unit includes logic that allows the compression controller to respond differently to periodic mechanical disturbances than to random mechanical disturbances. For example, based on information from the acceleration detection unit, the compression controller can identify when periodic mechanical disturbances are occurring and subsequently adjust the compression ratio of the compression unit in advance of succeeding periodic mechanical disturbances. If the disturbances cease to occur in a periodic fashion, then the compression controller can indicate to the compression unit to return to a standard compression ratio. In contrast, the compression controller can, in conjunction with the acceleration detection unit, identify when a random mechanical disturbance has occurred that requires the compression ratio of the compression unit to be adjusted. Since there is no way of predicting when a random mechanical disturbance will occur or how long the random disturbance will last, the compression ratio is adjusted over a time interval that is subsequent to at least the start of the mechanical disturbance.
In an enhanced version of the digital video system, the compression controller receives storage level information from the write buffer, and this information is used in addition to the acceleration detection unit information to adjust the compression ratio of the compression unit. That is, if the write buffer indicates to the compression controller that the write buffer is near its storage capacity, then the compression controller can increase the compression ratio achieved by the compression unit in order to reduce the incoming flow of write compressed digital video data to the write buffer and therefore preserve the uninterrupted digital video stream.
An advantage of the invention is that uninterrupted digital video streams can be recorded even when data writing is temporarily suspended without requiring expensive modifications to a digital video system, such as adding large amounts of RAM or increasing mechanical stability.